The applicant developed a sensor for measuring positions of a moving object such as a traveling vehicle, a transportation apparatus, a transfer apparatus, a head of a working machine or a workpiece (e.g., Patent Publication 1: JP2009-2660A). This position detector uses a combination of magnetizable material and non-magnetizable material or a magnet or the like as a mark, and uses coil arrays to determine positions based on the mark. That is, phases of signals outputted from coil arrays are different depending on the relative position to the mark, and the difference are used to determine the position based on the mark. Further, in the case where a plurality of marks are present, an offset is stored for each of the marks, and the offset is added to the position based on the mark to determine an absolute position.
Next, for example, a case where a range of measuring the position is large will be considered. It is assumed that a movement stroke of a moving object, or the entire length of a travel route is large. In order to continuously detect the position in the case where the measurement range is large, it is effective to provide a pair of position detectors at least on left and right sides of the moving object, provide the marks on left and right sides of the moving route of the moving object, and detect the marks using any of the position detectors all the time (Patent Publication 2: JP2008-140144A). However, the inventor found that, at the time of switching between a pair of left and right position detectors, the indicator value indicating the position of the moving object changes discontinuously. If the position changes discontinuously, large noises will be inputted to a servo system operated based on the indicator value from the position detectors disadvantageously.